The present invention relates to a crane vehicle mounted with a crane for lifting and conveying a weight load, and more specifically relates to control of the crane operation speed in order to prevent overturn of the vehicle body.
Conventionally, the crane vehicle has been frequently used to carry a crane mounted on a body of the vehicle to a working spot, where the crane vehicle is held stationary and the crane is operated to carry out loading or unloading of a weight load. Such type of crane vehicle is provided with a plurality of outriggers expandable outwardly widthwise of the vehicle body in the opposite directions so as to bear the vehicle body against the ground during the operation of the crane in the working spot to thereby support the weight load applied to the vehicle body through the crane. The outriggers are utilized to fix the vehicle body in place and to avoid inclination and overturn of the vehicle body.
Normally, the crane is comprised of a column erected on the vehicle body and turnable around its vertical axis, and an arm pivotably mounted at its bottom end on the top of the column such that the arm undergoes angularly elevating movement, and provided at its free top end with a hook for lifting a weight load. Further, the arm is expandable linearly along its axis. By such construction, the crane can be operated to convey three dimensionally the lifted weight load around the working spot. During the three dimensional operation of crane, the center of gravity of the combined mass of vehicle body, crane and weight load is shifted according to the current position of the arm top end. Thus, if the arm top end is applied with a relatively heavy load and is operated to reach far away from the vehicle body center, the center of gravity of the combined mass may be deviated beyond the span of the outriggers, to thereby cause inclination and overturn of the vehicle body.
In view of the above described possibility of inclination and overturn, some types of crane vehicles are provided with a sensor for monitoring unbalance of the weight load components applied between opposite sides of the vehicle body to stop the operation of crane or to produce an alarm signal when the monitored unbalance exceeds a preset critical amount. However, such a simple control system is not practically effective to perfectly prevent overturn of the vehicle body under various conditions of crane operation and various circumstances of working spots. The vehicle body might overturn due to a sudden stop operation of the crane. The vehicle body might overturn before the monitored unbalance exceeds the preset critical amount under the worst condition and circumstance. The operator might ignore the alarm signal.